Hydrostatic-mechanical drive system

ABSTRACT

Hydrostatic-mechanical gearbox for agricultural and construction vehicles with adjustable reducing gear ratio and a spring-loaded disk brake (19) incorporated in the mechanical gearbox (9) and which acts directly upon the driven shaft (11) of the mechanical gearbox (9) and is actuated depending on the operational condition of the hydrostatic gearbox.

FIELD OF THE INVENTION

The invention relates to a hydrostatic-mechanical drive system and amethod for the operation thereof.

THE RELATED ART

Comfortable driving and operational safety of modern vehicles carryingagricultural and construction machines require that the vehicle bedrivable in various speed ranges according to traffic, that it can besafely parked, and that it can be ready to perform these functions witha high degree of reliability. The transmission and the brake componentsof agricultural and construction vehicles are highly exposed to thedangers of dirt and damage. The complex functions which have to beperformed by modern agricultural and construction vehicles require theoperation of a great number of regulating units and increase thepossibility of human error during the operation of these vehicles.

It is known (DE-OS No. 34 25 757) to have a hydrostatic-mechanical drivesystem equipped with an adjustable reducing gear ratio, so that theregulating elements shift the adjustable hydrostatic gearboxautomatically, depending on changes in the reducing ratio of amechanical gearbox.

Further, from DE-A-No. 31 49 473 a hydrostatic drive system has becomeknown, which has a hydraulic motor with variable delivery and wherein aspring-loaded parking brake triggered by the pressure medium from thehigh-pressure segment is mounted on the driven shaft of the hydraulicmotor. If the pressure of the pressure medium surpasses a certain value,the hydraulic motor is reset in a position for increased delivery,whereby for the braking of the vehicle a high braking force is supposedto be produced.

SUMMARY OF THE INVENTION

It is the object of the present invention to create an operationallysafe combination of a hydrostatic-mechanical multirange drive systemwith a mechanical parking brake and to make the control of thehydrostatic-mechanical multirange drive and of the parking brakeespecially user-friendly.

According to the invention, this objective is achieved by incorporatinga mechanical parking brake into a mechanical two-step gearbox, theparking brake acting directly upon the driven shaft of the mechanicaltwo-step gearbox, and by the fact that for the actuation of the parkingbrake pressure medium is branched off from the high-pressure segment ofthe hydrostatic cycle via a pressure duct and that a 3/2-way valve isswitched which actuates a cylinder of the parking brake with thepressure of a servo hydraulic pump of the hydrostatic gearbox orconnects it to a pressure-free return duct.

Since the parking brake acts directly upon the driven shaft of thetwo-step mechanical gearbox, when the brake is engaged the vehicle isprevented from unintentionally rolling away, independently of the factthat one speed is engaged. Because the parking brake is incorporated inthe mechanical gearbox housing , it is possible to achieve aparticularly compact construction of the gearbox and the parking brake,affording considerably improved safety for the parking brake againstdamage or dirt.

Since the actuation of the parking brake depends on the operationalcondition of the hydrostatic gearbox, a particularly safe operation ofthe parking brake is insured and for the driver the operation of thevehicle is simplified by one regulated quantity.

According to the invention, the arrangement of the mechanical gearboxwith parking brake and hydrostatic drive and the actuation of theregulating unit for the adjustable hydromotor and the shifting of themechanical two-step gearbox with the same hydraulic pressure medium usedfor the actuation of the parking brake lead to a particularlyoperationally safe drive system and method for controlling such a drivesystem.

BRIEF DESCRIPTION OF THE DRAWING

In the FIGS. 1 to 3, embodiment examples of the invention arerepresented.

DETAILED DESCRIPTION

FIG. 1 is a general view of a drive system of a vehicle. It comprises adrive engine 1, which via a drive shaft 2 drives a servo hydraulic pump3 for the production of servo pressure and via a shaft extension 2'drives a hydraulic pump 4 with adjustable delivery. The hydraulic pump 4is connected via working ducts 5 and 6 to a hydraulic motor 7 withadjustable throughput. Over a drive shaft 8, the hydraulic motor 7 isconnected with a mechanical two-step gearbox 9. Depending on a shiftring operated in the known manner, the rotational speed of the drivenshaft 11 is determined by the reducing ratio of two pairs of spur gears12/13 or 14/25.

The driven shaft 11 drives two half axles 17, 18 via a differential 16.In the gearbox 9, a spring-loaded disk brake 19 is mounted on the drivenshaft 11. A spring-loaded piston 20 in a cylinder 21 is connected withthe disk brake 19.

In the cylinder 21 ends a duct 22, which is connected to the servopump 3or a pressure-free return pass 25 via a 3/2 way valve 23 controlled byhigh pressure and a duct 24. The control of the high-pressure 3/2 wayvalve 23 is connected via pressure ducts 26, 27 to the working ducts 5,6.

A two-position setting mechanism 28 with a piston 29 in a cylinder 30regulates the hydraulic motor 7. The cylinder 30 is connected toservopressure or to the pressure-free return pass via 3/2-way valve 31and the duct 32. The actuation of the 3/2-way valve 31 takes place inthe usual manner, by electromagnet or pressure.

The mechanical two-step gearbox is hydraulically shifted. In a cylinder33, a piston 34 is alternately actuated at one or the other of its endswith servopressure. A 4/3-way valve 35 connects one side of the pistonwith the servopressure and the other side of the piston with thepressure-free return pass, depending on the position of the piston. Apressure-relief valve 36 protects the servopressure duct system againstexcessive loads.

FIG. 2: The hydromotor 7 is flanged in the known manner to themechanical two-step gearbox 9. The drive shaft 8 of the hydromotor 7drives the driven shaft 11 depending on the position of the shift ring10, and thereby also the half axles 17 and 18 with variable rotationalspeed. On the driven shaft 11, the disk brake 19 is mounted. The piston20 and the cylinder 21 are connected via duct 22 with the 3/2-way valve23, actuated by high pressure. A prestressed spring 37 also acts uponthe piston 20, pressing it against the disks 38 of the disk brake 19.

FIG. 3: The duct 24 can be dispensed with, by connecting the pressureconnections of both 3/2-way valves 23, 31 with the pressure duct 26.Thereby, the pressure duct system is considerably simplified.

Description of Function

When the engine 1 is stopped, the servopressure system and the ducts 5,6, 27 are pressure-free. It results that the pressure actuated 3/2-wayvalve 23 is in the position shown in FIG. 1, and the cylinder 21 isconnected with the pressure-free pass 25. The disk brake 19 is pressedtogether by a spring and this way the vehicle is prevented from rollingaway.

After the engine 1 has been started, the servopump 3 builds up theservopressure. As soon as the pump 4 has been adjusted for delivery, theoperational pressure of the hydrostatic gearbox is present in the ducts5 or 6, as well as in pressure ducts 27, 26. The operational pressuresets the 3/2-way valve so that the duct 24 is in connection with theduct 22 and the servopressure acts upon the piston 20, causing thepiston 20 to move against the spring pressure and the parking brake 19is vented.

Since the hydraulic pressure medium also drives the hydromotor 7, thevehicle can be set in motion corresponding to the gear ratio of thehydrostatic and the mechanical gearbox, to be selected by the driver.

We claim:
 1. A hydrostatic-mechanical drive system particularly foragricultural and construction vehicles, said system comprising:anadjustable hydrostatic gearbox including a servohydraulic pump, saidgearbox employing for control a hydraulic pressure medium operating in ahydrostatic cycle, said cycle including a high-pressure segment; amechanical two step gearbox subsequently serially connected to saidadjustable hydrostatic gearbox, comprising: a driven shaft; and amechanical parking brake acting directly upon said driven shaft, saidbrake including a cylinder; a pressure duct leading away from saidadjustable hydrostatic gearbox; a 3/2-way valve communicating with saidpressure duct and said cylinder; a pressure-free return ductcommunicating with said 3/2-way valve and said pressure duct; andwherein said parking brake is actuated by a branching off of saidpressure medium from said high-pressure segment of said hydrostaticcycle through said pressure duct to switch said 3/2-way valve eithercausing actuation of said cylinder of said parking brake with pressurefrom said servohydraulic pump or causing connection of said parkingbrake with said pressure-free return valve.
 2. A hydrostatic-mechanicaldrive system according to claim 1, said adjustable hydrostatic gearboxfurther comprising an adjustable hydromotor, and said system furthercomprising controls for controlling said adjustable hydromotor and saidmechanical two-step gearbox, said controls being actuated with saidpressure medium in response to a signal, said signal being independentof any operational condition of said hydrostatic gearbox.
 3. Ahydrostatic-mechanical drive system according to claim 1, wherein saidmechanical parking brake further comprises a prestressed spring, saidbrake being actuated by said prestressed spring in a closing directionand being vented due to said hydraulic pressure medium of saidhydrostatic gearbox.